The present invention relates to apparatus for providing magnetic fields and, more particularly, to a magnet system for providing a region of substantially homogeneous magnetic field strength.
There are many areas of application for a region of substantially homogeneous magnetic field strength. An area of application is in equipment making use of the phenomenon of nuclear magnetic resonance. One such piece of equipment for medical diagnosis provides information comparable to that provided by a computerized axial tomography scanner but without subjecting the patient undergoing examination to potentially harmful X-rays.
It has been recognized that the magnetic field provided by a practical solenoid coil is not sufficiently homogeneous for such applications. In order to correct for nonuniformity in field strength, correction coil windings, connected to a source of current, have been employed at predetermined locations along the length of the solenoid. Lengthy computations as to ampere turns and extremely precise coil placement are required in order to achieve the desired field homogeneity. Slight errors in the construction of the solenoid coil obviously cannot be taken into account in the determination of the configuration and placement of the correction coils, and such slight errors could result in the region of desired homogeneity exhibiting unacceptable variations in field strength. Another problem with prior correcting coil windings is that the ambient magnetic field environment, once defined and taken into consideration in the design of the coils, cannot be altered without degradation in the field homogeneity. For example, rearrangement of furniture having ferromagnetic materials or movement of an elevator in the vicinity of the solenoid coil would degrade the desired homogeneity.
A previously proposed toroidal field-generating coil system employs two types of coils with each type of coil having sections. The number of turns and current magnitude are selected to insure the magnetic field strength at certain sections of the first type coils is approximately the same as at certain sections of the second type coils. Current is passed in opposite directions in preselected adjacent sections of different type coils so that their respective magnetic fields offset one another. For a further description of the structure and operation of such magnet system, reference may be made to U.S. Pat. No. 4,263,096.
In another prior art magnet, a superconducting solenoid is employed having a field-shaping member made of a metal having high magnetic saturation. The field-shaping member functions to prevent production of localized regions of high magnetic field strength at inner points of the superconducting winding. The field shaper interacts with the magnetic field lines in the localized region and directs the field lines to form a resultant substantially uniform strong field along the axis of the solenoid. Reference may be made to U.S. Pat. No. 3,177,408 for a further description of the structure and operation of this magnet.